Approximately five and three quarter million shipping containers a year, or ninety-five percent of all international origin goods, arrive in the U.S. by sea. At U.S. ports, inspecting a 20 to 40 foot long shipping container can take four customs inspectors about four hours. At that rate, often fewer than two percent are opened for inspection, and the great majority never pass through any existing sensors, e.g., x-ray machines, gamma-ray probes, or the like. In addition, U.S. borders to the north and south accommodate 125 million vehicles (including 11.2 million trucks), 2.2 million rail cars and 500 million people on an annual basis. Significantly, such prior art sensors are essentially blind to biological hazards. Front line inspectors, customs officers and other law enforcement officers have consequently called for new technology to screen shipments, whether from air, ship or rail, so that time consuming and labor intensive searches can be minimized and dangerous cargo can be prevented from entering the U.S.
Other applications include First Responders (EMT's, fire departments, and law enforcement agencies) that would benefit from an all-inclusive device to assess contaminated areas or detect hazardous materials. In addition, the device could be used for the inspection of air handling systems for “sick building syndrome” or Legionaries′ disease and toxic mold in commercial and residential buildings.
There are many substances which have very small vapor pressures, but whose presence in air is nonetheless undesirable because they are very toxic or indicate the presence of unwanted substances hazardous chemicals, biological agents, explosives, drugs, etc. (hereinafter referred to as “BCA's” or Biological, Chemical Agents). Current detection methods for BCA hazards are extremely slow, and are often based upon complicated chemical or mechanical concepts, use of multi-step and labor intensive approaches, or the need for replaceable supplies (consumables). In addition, such prior art devices are often very large (not portable) and expensive.
The saturation concentrations of these hazardous substances in air at room temperature suggest that they can be detected using existing techniques. However, in the real world, they are unlikely to be presented to a detector with a sufficient volume of saturated air to make such detection easy. At best, the fraction of molecules available to a ‘sniffer’ will be reduced by a few orders of magnitude. Therefore, sensors must be able to detect these materials at vapor concentrations a few orders of magnitude less than their saturation concentrations.
It is well known in the art to use the anti-electron (commonly referred to as a “positron”) to probe the structure of molecules. This field owes most of its existence to the study of the crystalline structure of semiconductor materials and the structure of polymers, e.g., isolation of irregularities in semiconductors and polymers. It is known that positrons of cosmic origin annihilate with extremely dilute molecular gases in interstellar space. Gamma rays that have been captured and recorded by satellites orbiting the Earth provide evidence for the existence of gases in incredibly small concentrations, and can even distinguish among various species of molecules. This technology has been further explored academically, for example, by K. Iwata et al., in their publication entitled: “Measurements of positron-annihilation rates on molecules”, Physical Review A 51, 473, 1995, which publication is hereby incorporated herein by reference.
The foregoing positron annihilation method generally comprises a process in which a positron is injected into physical matter from a positron source, and the lifetime of the positron (i.e., the time between injection and annihilation) is measured to indirectly determine various characteristics of the matter. A positron is the anti-particle of an electron, and is an elementary particle having the same mass and the opposite charge as an electron. When positrons are implanted in a solid they are rapidly thermalized and annihilate with electrons. It is known that a positron and an electron briefly form an electron-positron pair (via coulomb forces) when the two particles meet in a molecular crystal or in an amorphous solid material, and then the pair annihilates. The positron-electron pair behaves in a manner similar to a particle in a bound state, and is referred to as “positronium.”
When positronium annihilates, two or three annihilation gamma-rays are emitted. There are two types of positronium, para-positronium and ortho-positronium. The spins of the electron and the positron are anti-parallel in the para-positronium and parallel in the ortho-positronium. Para-positronium decays into two 511 kiloelectronvolt (keV) gamma rays, one in each of two directions with an angel of 180° between them. Ortho-positronium decays into three gamma rays, the sum energy of which is 1022 keV. While the lifetime of a para-positronium pair is about 0.13 nanoseconds (ns), the lifetime of an ortho-positronium pair depends upon the electron density in the surroundings of the positronium. The mean lifetime of ortho-positronium in vacuum is about 140 ns, when it is annihilated in a self-annihilation process. However, the lifetime decreases down to the range from about 1 to about 5 ns when an ortho-positronium pair annihilates through a “pick-off” process in which the positronium takes electrons from the surrounding matter. With the aforementioned positron annihilation method, a positron lifetime is determined by measuring the time variation in intensity of the annihilation gamma-rays emanating from the material into which the positrons had been injected.
The use of ortho-positronium decay is known for the determination of the location and size of crystal lattice defects. For example, when ortho-positronium exists in a vacancy-type defect, the measured lifetime of the ortho-positronium correlates well with the size of the defect. With increases in the size of the vacancy-type defect, the probability that the ortho-positronium will succumb to “pick-off” annihilation with an electron oozed out from the inner wall of the defect decreases, resulting in longer lifetimes of the ortho-positronium. Thus, the size of the defect can be determined by measuring the lifetime of the ortho-positronium. It is also known, however, that the lifetime of ortho-positronium tends to saturate when the radius of the defect increases beyond a certain value, e.g., about 0.5 nanometers (nm) so that the maximum value of the radius of a defect measurable by this method is about 0.5 nm.
There is a need in the art for an improved method and apparatus for sensing and monitoring the ingress of so called hazardous BCA materials into the United States of America. It would be of benefit if the foregoing positron annihilation method could be used to detect such hazardous BCA materials.